1. Field of Invention
The present invention relates to the field of cutting and sealing sheets of polymers. More particularly, this invention relates to cutting and sealing sheets of polymers such as the polyethylenes and polyolefins commonly used in products such as bubble film, bread bags and freezer bags.
2. Description of the Related Art
Cutter-sealers that use heat to cut and form a seal between the edges of sheets of polymer are well known in the art. Typically, a heated blade is brought to bear on two sheets of a polymer supported by a rubber seal pad. The heated blade melts the polymer and divides it in two to push liquefied polymer to each side of the blade. The liquefied polymer pushed to each side of the blade seals the edges of the newly divided polymer together. Thus the typical polymer cutter-sealer uses heat to melt the polymer so as to both divide and seal in a single operation.
The use of melting heat to both cut and seal is possible because of the unique properties of polymers, which are simply substances whose molecules have high molar masses and are composed of a large number of repeating units. Polymers are generally formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. Plastic is the most common example of a polymer. A polymer's reaction to heat is determined by the molecules used as monomers and the structure of the chain. Polyethylenes, for example, are a subset of polymers in which the chains are formed by a double bond between carbon atoms. Polyethylenes are commonly used for padded packaging such as bubble film, which is made from sheets of materials that are sealed together to form a single layer of packaging having an array of air bubbles formed between the layers. Such bubble film is often cut to a desired length and sealed together to form the closed end of a container in an operation similar to that described above.
Obviously, the speed and quality with which polymer film containers can be produced is a significant factor in a competitive marketplace. Hence, several machines and techniques have been developed that focus on this particular aspect of the manufacturing process. Many of these, including the device disclosed in U.S. Pat. No. 5,056,295, issued to Williams, use heat to melt the polymer in order to both divide and seal. However, the use of melting heat simultaneously for both dividing and sealing operations forces conventional cutter-sealer blades to be heated to temperatures well above the melting point of the polymer film so that the necessary liquefaction occurs. The liquefaction of polymers, however, creates residue on the blade, which can stop production for removal of the residue or cause incomplete cuts and faulty seals if left unchecked. The high temperatures that are necessary also make it difficult for human operators to interact with the process.
The blade design dictated by the use of melting heat to cut and seal layers of polymer film often produces a thin or impartial seal. For example, in several designs, only the material pushed to the sides of the blade creates the seal. In such designs, the conventional blade may not push equal amounts of material to both sides, creating a thinner seal than normal on one side and more residue that normal on the other.
In other designs, such as U.S. Pat. No. 5,131,213, issued to Shanklin et al., a heating apparatus contacts a portion of the polymer layers, applying heat to the layers in order to seal a portion of the layers. Thereafter, a blade contacts the sealed layers to cut the sealed portion of the layers. In such designs, again the problem arises of exposing the cutting blade to a section of heated layers. In such designs, the initial heating of multiple polymer film layers followed by cutting of the heated layers often results in excessive buildup of melted polymer residue on the cutting blade, which can stop production for removal of the residue or cause incomplete cuts and faulty seals if left unchecked. In the case of sealing and cutting bubble film, such initial heating of multiple layers of bubble film can also result in overheating and explosive rupturing of the portion of the bubble array to be sealed and cut, thereby resulting in irregular and partial sealing of the bubble film layers.